1. Field of the Invention
This invention relates to a coupling control device and more particularly to a fluid coupling control device of an automatic transmission.
2. Description of the Related Art
A conventional fluid coupling control device of an automatic transmission is disclosed in Japanese Patent Laid Open No. 62(1987)-200070. The fluid coupling control device disclosed in this document comprises a turbine chamber into which an operation fluid flows, with a driving power being transmitted through the operation fluid from an input member to an output member, an operation chamber which is separated from the turbine chamber and into which the operation fluid flows, a clutch mechanism which engages and disengages the input member with the output member, a piston member slid by a fluid pressure differential between the operation chamber and the turbine chamber so as to engage and disengage the clutch mechanism, a fluid source which supplies the fluid to the operation chamber and the turbine chamber, and a control device disposed between the fluid resource and the operation chamber which controls the supply of fluid to the operation chamber and the fluid drain from the operation chamber. The control device further includes an entrance port into which the fluid flows, a passage which introduces the fluid into the operation chamber from the fluid source, and a spool valve which opens and closes the passage and which is moved by the fluid so as to supply the fluid to the operation chamber and drain the fluid from the operation chamber.
Since the fluid which flows into the turbine chamber is used for cooling elements of the automatic transmission, the pressure in the turbine chamber varies depending upon the number of elements and conduits connecting the elements. Therefore, an engaging force of the clutch mechanism also varies because the piston member is moved by the fluid pressure differential between the operation chamber and the turbine chamber so as to engage and disengage the clutch mechanism. Consequently, a large shock is generated in the clutch mechanism when the clutch mechanism is engaged.